1. Field of the Invention
The field of the invention is that of systems of cellular radio communications, notably for communications with mobile units. More specifically, the invention relates to the laying out of the infrastructure of cellular networks, i.e. particularly the determining of the geographical location of the transmitters and of their technical characteristics so as to achieve optimum compliance with a certain number of constraints (such as geographical coverage, bit rate etc.).
The method of the invention can be applied to any type of cellular network. A especially promising field of application is that of radiotelephony with mobile units, whether these are analog systems such as the "Radiotom 2000 Haute Density" (registered mark) system or digital systems such as the European GSM (Group Special Mobile) system.
In a simplified version, the invention can also be applied to digital broadcasting systems such as the DAB (digital audio broadcasting) system.
Hereinafter, and without thereby restricting the scope of the invention, it is essentially the case of radiotelephony that will be considered. Indeed, an efficient solution to the problem of the growing number of radiotelephone subscribers is the introduction of a cellular-architecture based system of radio communications with the mobile units.
2. Description of the Prior Art
In such systems, the territory to be covered is cut up into zones or sectors known as cells.
Each cell is served by a base station (transmitter, receiver) fulfilling the dual function of ensuring radioelectric or radio coverage and of handling the communications traffic generated by the mobile units that are linked to it by radio.
Two contiguous cells use different frequencies, but the frequency assigned to a cell can be re-used further down, provided that rules of re-use proper to the system are followed, ensuring a certain degree of protection from interference. The set of connex (neighboring) cells sharing the totality of the frequencies allocated to the network is called a cell pattern.
It is in the interest of cell networks to enable the re-use of the frequencies, by translation of the cell pattern. The trade-off here is that it becomes necessary to track the mobile units. It is necessary to have the ability to localize the cell visited by a mobile unit or, more generally, the group of cells (zone of localization) to which this cell belongs, to enable a call to be transferred to this mobile unit, but it is also necessary to have the ability to maintain communications that are in progress when changing cells (i.e. to achieve intercell handover).
While, until now, the dimensions of the networks enabled a pragmatic approach to certain technical questions, the present complexity of these networks makes it indispensable to set up a tool of assistance to the engineering of cellular networks.
The planning of the mobile cellular networks consists chiefly in:
--deciding the locations for the installation of the transmitter stations in ensuring appropriate coverage from these sites; PA1 --defining the cell pattern, and distributing the frequencies; PA1 --dimensioning the cells in terms of traffic channels and signalling; PA1 --defining the systems parameters such as the localization zones, PA1 --determining the parameters of the algorithms used in the networks (for the inter-cell handover for example) taking account notably of the following optimization criteria: PA1 --radioelectric or radio resources (Erlang/MHz/km2), PA1 --infrastructural costs (dimensioning of traffic and signalling channels, size of cells etc.). PA1 --spectral efficiency; PA1 --telephone traffic and signalling density of the network and, consequently, the optimizing of infrastructural costs. PA1 --determining the radio coverage of at least certain of said broadcasting cells, PA1 --determining the traffic, i.e. exchanges of information, in at least certain of said broadcasting cells, taking account of said method for the assigning of cells. PA1 --automatic inter-cell handover for mobile units that are in the process of communicating and changing their broadcasting cell; PA1 --reselection of a cell, for mobile units changing from an off state to a stand-by watch state or from a stand-by watch state to a communicating state, so as to enable the transmission or reception of information elements. The reselected cell is generally the cell that provides the best conditions of reception. PA1 --starting point of said mobile unit; PA1 --speed of said mobile unit; PA1 --law governing the movement of said mobile unit; PA1 --direction of said mobile unit; PA1 --state of the mobile unit (in communication mode, in stand-by watch mode, out of stand-by watch mode (or at rest)). PA1 --measurements of radioelectric field; PA1 --interpolation of the radio coverage from said measurements, to geographical zones close to said measurements; PA1 --extrapolation of said radio coverage from said measurements; PA1 --choice of a propagation model from among at least two available models, and an operation to determine the total radio coverage, on the basis of said measurements and/or interpolations and/or said extrapolations made and/or the propagation model chosen. PA1 --classification of the measurements into at least two groups; PA1 --application of a distinct model of interpolation for each of said groups. PA1 --antenna pattern; PA1 --power; PA1 --azimuth; PA1 --number and/or value of the transmission frequencies; PA1 --number of service channels; PA1 --number of communications channels. PA1 --predefined location of a transmitter and/or receiver; PA1 --relief; PA1 --predefined form of at least one cell.
There already exist known procedures for the planning of cell networks, such as those proposed by the following systems: GRAND (registered mark), PHILIPS (registered mark) or CELLNET (registered mark) by BRITISH TELECOM.
These procedures are essentially oriented towards the planning of radio projects. A probabilistic model of radio wave propagation is used in a region for which a plan is to be prepared. The values of the parameters of this model are obtained by the standard method of statistical regression on the basis of field measurements. It is then possible to determine the radio coverage of the network for this model.
Certain tools also take account of the traffic aspects. However, in this case, this is done only statically: all that these tools process is the integration of distribution of traffic measured. This entails the assumption that the network is already operational, and permits only second-level modifications. If a base station is entrusted with managing a flow of traffic that is excessive for its configuration, then the engineering has to be modified locally.
Besides, in these existing tools, the procedure used for making the final adjustments to the system always entails the assumption that the coverages used are connex and/or convex and therefore do not show any interruptions or uneven features as far as the user is concerned. Now experience shows precisely that, especially in an urban environment, the coverages are accompanied by gaps and phenomena of resurgence that are detrimental to the quality of service.
It is an aim of the invention notably to overcome these drawbacks of the prior art.
In particular, an aim of the invention is to provide a method for the laying out of the infrastructure of a cellular network enabling the optimization of the transmitter sites from several points of view, notably those of:
Another aim of the invention is to provide a method such as this that can be used both for the complete building of a cellular network, it being then possible to compare several simulated options if necessary, and for the development of an already existing network (i.e. for increasing its capacity and/or its geographical coverage) or for the real-time or deferred follow-up of an existing network.
The invention is also aimed at providing a system such as this that can apply to any cellular system, whether the broadcasting is analog or digital and whether the broadcasting is one-directional or radiotelephonic (two-directional).
A secondary aim is to provide a method such as this that optimizes the sharing and use of the total transmission resources.
Another secondary aim of the invention is to provide a method such as this that also provides for the optimization of the automatic inter-cell handover and reselection algorithms (which shall be described in greater detail here below) implemented by the management system of the network.